是否可以将ObjectProperty内的ObservableList的非空状态与Bindings API绑定？

Question

我有一种情况,我想绑定BooleanProperty到一个ObservableList包裹在一个非空状态ObjectProperty.

这是我正在寻找的行为的基本概要:

    ObjectProperty<ObservableList<String>> obp = new SimpleObjectProperty<ObservableList<String>>();

    BooleanProperty hasStuff = new SimpleBooleanProperty();

    hasStuff.bind(/* What goes here?? */);

    // ObservableProperty has null value 
    assertFalse(hasStuff.getValue());

    obp.set(FXCollections.<String>observableArrayList());

    // ObservableProperty is no longer null, but the list has not contents.
    assertFalse(hasStuff.getValue());

    obp.get().add("Thing");

    // List now has something in it, so hasStuff should be true
    assertTrue(hasStuff.getValue());

    obp.get().clear();

    // List is now empty.
    assertFalse(hasStuff.getValue());

我想在类中使用构建器,Bindings而不是实现一系列自定义绑定.

该Bindings.select(...)方法理论上做了我想要的,除了没有Bindings.selectObservableCollection(...)并且从通用中转换返回值 select(...)并将其传递给Bindings.isEmpty(...)不起作用.也就是说,结果如下:

    hasStuff.bind(Bindings.isEmpty((ObservableList<String>) Bindings.select(obp, "value")));

导致ClassCastException:

java.lang.ClassCastException: com.sun.javafx.binding.SelectBinding$AsObject cannot be cast to javafx.collections.ObservableList

这个用例是否可以仅使用BindingsAPI？

---

解

根据@fabian的回答,这是有效的解决方案:

    ObjectProperty<ObservableList<String>> obp = new SimpleObjectProperty<ObservableList<String>>();

    ListProperty<String> lstProp = new SimpleListProperty<>();
    lstProp.bind(obp);

    BooleanProperty hasStuff = new SimpleBooleanProperty();
    hasStuff.bind(not(lstProp.emptyProperty()));

    assertFalse(hasStuff.getValue());

    obp.set(FXCollections.<String>observableArrayList());

    assertFalse(hasStuff.getValue());

    obp.get().add("Thing");

    assertTrue(hasStuff.getValue());

    obp.get().clear();

    assertFalse(hasStuff.getValue());

Answer 1

我没有看到使用Bindings API的方法.ObservableList没有属性为空,因此您无法使用

Bindings.select(obp, "empty").isEqualTo(true)

和

ObjectBinding<ObservableList<String>> lstBinding = Bindings.select(obp);
hasStuff.bind(lstBinding.isNotNull().and(lstBinding.isNotEqualTo(Collections.EMPTY_LIST)));

不起作用,因为它只在列表更改时更新,而不是在内容更改时更新(即第三个断言失败).

但是您必须创建的自定义绑定链非常简单:

SimpleListProperty lstProp = new SimpleListProperty();
lstProp.bind(obp);
hasStuff.bind(lstProp.emptyProperty());